In present day communication systems there are many transmission media which contain both memoryful elements and non-linear elements that undesirably affect the fidelity of information passing through the media and require the use of error or distortion correction circuitry to overcome the influence of these factors on the received signal.
An example of such a medium is the telephone channel which contains both linear filters and non-linear elements that produce harmonic distortion. When high rate digital signals are transmitted through such a channel, the combination of the disturbances produced by each of these elements degrades the error rate of the received signal in such a manner that cannot be accomodated well by presently available processing techniques.